1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an array substrate for a liquid crystal display device, which includes an organic semiconductor layer as a switching element being capable of reducing contact resistance between the organic semiconductor layer and each of the source and drain electrodes, and a method of fabricating the same.
2. Discussion of the Related Art
Recently, as society has entered into an information age, a field of display devices that represents all sorts of electrical signals as visual images has developed rapidly. Moreover, since the LCD device is light weight, thin, and requires low power consumption, the LCD device has been widely used as a substitute for a cathode-ray tube type display device.
A related art liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite alignment direction as a result of their thin and long shapes. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field across the liquid crystal molecules. In other words, as the intensity or direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Since incident light is refracted based on the orientation of the liquid crystal molecules due to the optical anisotropy of the liquid crystal molecules, images can be displayed by controlling light transmissivity.
Since the LCD device including a thin film transistor (TFT) as a switching element, referred to as an active matrix LCD (AM-LCD) device, has excellent characteristics of high resolution and displaying moving images, the AM-LCD device has been widely used.
FIG. 1 is an exploded perspective view of a related art liquid crystal panel. As shown in FIG. 1, the liquid crystal panel includes an array substrate 10, a color filter substrate 20, and a liquid crystal layer 30. The array substrate 10 and the color filter substrate 20 face each other, and the liquid crystal layer 30 is interposed therebetween.
The array substrate 10 includes a first substrate 12, a gate line 14, a data line 16, a thin film transistor (TFT) Tr, and a pixel electrode 18. The gate and data lines 14 and 16 are formed on the first substrate 12 and cross each other to define a pixel region P. The TFT Tr is formed at a crossing portion of the gate and data lines 14 and 16. The pixel electrode 18 is formed in the pixel region P and connected to the TFT Tr.
The color filter substrate 20 includes a second substrate 22, a black matrix 25, a color filter layer 26, and a common electrode 28. The black matrix 25 is formed on the second substrate 22 and has a lattice shape. The black matrix 25 corresponds to a non-display region of the first substrate 12. The non-display region of the first substrate 12 includes the gate and data lines 14 and 16 and the TFT Tr. The color filter layer 26 corresponds to the pixel region P and includes red, green, and blue color filter patterns 26a, 26b, and 26c. The common electrode 28 is formed on the black matrix 25 and the color filter layer 28. The common electrode 28 generates an electric field with the pixel electrode 18 such that the liquid crystal layer 30 is driven by the electric field.
Though not shown, a seal pattern is formed along edges of the first and second substrates 12 and 22. The seal pattern prevents the liquid crystal layer 30 from overflowing. In addition, first and second alignment layers may be formed between the first substrate 12 and the liquid crystal layer 30 and between the second substrate 22 and the liquid crystal layer 30. A polarization plate may be formed on an outer surface of one of the first and second substrates 12 and 22. A backlight assembly is formed on a rear side of the first substrate 12 to apply light into the liquid crystal panel. When a scan signal is applied to the TFT Tr through the gate line 14 to turn on the TFT Tr, an image signal is applied to the pixel electrode 18 through the data line 16 such that an electric field is generated between the pixel electrode 18 and the common electrode 28. As a result, the liquid crystal molecules in the liquid crystal layer 30 are driven by the electric field to display images.
Generally, a glass plate is used for the first and second substrates 12 and 22. However, recently, a flexible plate, such as a plastic plate, is used for the first and second substrates 12 and 22 because the flexible plate is light and flexible.
Unfortunately, since a process of fabricating an array substrate is performed under a temperature higher than about 200° C., it is very difficult for the flexible plate to be a substitute for the glass plate. So, the array substrate is made of the glass substrate, and the color filter substrate is made of the flexible substrate. When processes of forming a metal layer, a gate insulating layer, a passivation layer are performed under a temperature lower than 200° C., the TFT does not deteriorate. However, when a semiconductor layer is made of amorphous silicon under such a lower temperature, the TFT does deteriorate. To resolve these problems, a method of fabricating the array substrate under a temperature lower than about 200° C. by forming the TFT using an organic semiconductor material is suggested.
Since the lower temperature process for the array substrate uses the coating apparatus, which is cheaper than a vacuum depositing apparatus, there is an advantage in the reduction of production costs. The lower temperature process is useful for not only the plastic substrate but also the glass substrate.
When processes of forming a metal layer, a gate insulating layer, and a passivation layer are performed at a temperature lower than 200° C., the TFT does not deteriorate. However, when a semiconductor layer is made of amorphous silicon under such a lower temperature, the TFT does deteriorate. To resolve these problems, a method of fabricating the array substrate at a temperature lower than about 200° C. by forming the TFT using an organic semiconductor material is suggested.
The TFT is classified into a top contact type and a bottom contact type depending on a contact relation between each of the source and drain electrodes and the organic semiconductor layer. In the top contact type TFT, the organic semiconductor layer is disposed on the source and drain electrodes such that a bottom surface of the organic semiconductor layer contacts a top surface of each of the source and drain electrodes. On the other hand, in the bottom contact type TFT, the source and drain electrodes are disposed on the organic semiconductor layer such that a top surface of the organic semiconductor layer contacts a bottom surface of each of the source and drain electrodes.
When the organic semiconductor material, which is formed through a coating process, is exposed to a developer for a photoresist or an etchant for etching a metal layer, a property of the semiconductor layer deteriorates. Accordingly, the semiconductor layer of the organic semiconductor material is formed after a source and drain electrodes forming process, since the source and drain electrodes forming process requires a patterning step using an etchant. That is, the bottom contact type is useful for the TFT using the organic semiconductor material.
Unfortunately, the bottom contact type organic TFT has a disadvantage; namely, a contact resistance between each of the source and drain electrodes and the organic semiconductor layer ends of the source and drain electrodes have a reverse-tapered shape. That is, a side surface of the ends of the source and drain electrodes have an obtuse angle with respect to a top surface of the substrate.
FIG. 2 is a picture showing an end of a source electrode in the related art TFT including an organic semiconductor layer.
As shown in FIG. 2, an end of a source electrode has a reverse-tapered shape with respect to a surface of a substrate. In this case, an organic semiconductor layer, which is formed on the source electrode by a coating apparatus, does not have a coating thickness uniformity. Particularly, the organic semiconductor layer has a larger thickness at the end of the source electrode such that a boundary region is generated in the organic semiconductor layer. As a result, there is a problem in properties, e.g., mobility, since the boundary region serves as barrier against movement of carriers. Accordingly, a contact resistance between each of the source and drain electrodes and the organic semiconductor layer is increased.
In addition, when the substrate, where the source and drain electrodes are formed, is transported for a process of forming the organic semiconductor layer, undesired particles may be attached into the reverse-tapered shape end of the source and drain electrodes. Particularly, the undesired particles in a space between the end of the source electrode or the drain electrode and the substrate are not removed by a cleaning process. Defects occur due to the undesired particles.